The present invention relates to an improved process for producing a magnetic head of a type used in a video tape recorder, digital audio tape recorder, or the like, and which is mounted at a selected location on the periphery of a drum which rotates as magnetic tape is guided past it to achieve helical recording or reproduction.
Recent technological advances toward higher packing densities in magnetic recording have been remarkable. This has caused a growing need to reduce the track width produce by magnetic heads both in recording and playback modes, as well as the width of the magnetic gap, leading to a corresponding increase in the closeness of dimensional tolerances. Taking 8-mm video tape recorders and digital audio tape recorders as examples, the magnetic head produces a track width of as small as about 10 microns and a gap width of 0.2 to 0.3 microns.
One of the currently most popular tape recorders of the type that employs magnetic heads mounted on a rotating drum is a home video tape recorder. The magnetic gap of a head in such a tape recorder, which makes sliding contact with magnetic tape, and the adjacent area to the gap are shown in FIG. 1. Shown by 1a and 1b are core block halves that combine together to form a magnetic circuit in the head; 2 is a glass material filled in track width controlling grooves formed in the mating surfaces of core halves 1a and 1b, the dimension of each core half between adjacent glass-filled grooves defining a track width; and 3 is a gap spacer inserted between the core halves 1a and 1b in the portion of the track width, with the thickness of this gap spacer providing a magnetic gap width.
A process for fabricating the magnetic head shown in FIG. 1 is described hereinafter with reference to FIG. 2. First, each of the core half 1b and the core half 1a, the latter being provided with a groove for accommodating a coil winding, is machined to form a plurality of track width controlling grooves, adjacent ones of which are spaced apart by a distance corresponding to the track width. After grinding and polishing the mating surfaces of the two core halves to a speculate finish, a gap spacer 3 whose thickness is half the magnetic gap width is deposited on each of the speculate surfaces. Subsequently, the core halves 1a and 1b are brought into abutment against each other in such a manner as to attain exact registry between opposite track forming portions. Each of the track width controlling grooves is then filled with a glass material 2 to form a core block of the shape shown in FIG. 2. This core block is cut along lines through the track width controlling grooves including the glass 2, thereby dividing the block into discrete magnetic heads, one of which is shown enlarged in FIG. 1.
The method described above with reference to FIG. 2 includes preparation- of a single core block containing a plurality of magnetic heads and cutting the block into discrete heads covering the area indicated by 5 in FIG. 2. This approach, however, has the disadvantage that a slight misalignment between the track portions to be brought into abutment against each other or irregularities n the pitch of the track width controlling grooves will cause the portion of the core half 1a defining the track width to be out of registry with the corresponding track width defining portion of the core half 1b, as shown in FIG. 3. If this situation occurs, only the area where the two core halves are in actual abutment against each other can provide an effective magnetic gap, and hence the track width is decreased. Such misalignment will cause a serious problem in a magnetic head having a small track width of only about 10 microns. In order to avoid such problems, very close tolerances are required in machining and joining the core halves 1a and 1b, but imposing such tolerances is unavoidably accompanied by a reduction in the production rate of the magnetic heads.
One approach that has been taken t prevent the occurrence of such problems is to cut track width controlling grooves in one core half (1b in the case shown in FIG. 4) in such a way that the area between adjacent grooves is wider than the desired track width. That core half is brought into abutment against the other core half 1a in which track width controlling grooves are cut to leave portions as wide as the intended track width. The advantage of this method is that the desired track width is attained even if there occurs a slight misalignment between the track portions of the two core halves that are brought into abutment against each other.
The fabrication of conventional heads including the one shown in FIG. 4 involves preparing a single block containing as many as several tens of magnetic heads as shown in FIG. 2. However, it is extremely difficult to attain a uniform gap length in all block halves by this approach, and the presence of even minute amounts of foreign matter or deviations from the prescribed dimensional precision in the gap surface of either block half will result in a gap length greater than the thickness of the nonmagnetic spacer. If the magnetic head to be produced is to have a small gap length, any deviation of gap length from the prescribed value will become a considerably significant factor.
As already mentioned, in tape recorders such as video and audio digital tape recorders, the magnetic heads are mounted on a rotating drum and magnetic tape wrapped partially around the drum is moved across the heads as the drum is rotated, thereby achieving "inclined azimuth" recording. If the magnetic head shown in FIG. 4 is used in this type of recording, the following problem will occur.
In helical-scan inclined-azimuth recording, the track width of a head is set to be greater than the pitch of recording/reproducing tracks, and the track width is determined by overwriting. In this case, the edge of each recorded track is determined by the end of the magnetic gap of a recording head. If the ends of the magnetic gap are in exact registry on both sides as shown in FIG. 1, a small magnetic flux will leak from these ends and the magnetic edge is exactly determined to produce recording patterns as shown in FIG. 5, wherein 62 denotes the region where information is recorded with a (+) azimuth head and 6a signifies the region where information is recorded with a (-) azimuth head. However, if information is recorded with the magnetic head shown in FIG. 4, a large flux leaks from the end of the magnetic gap rendering it indistinct since the flux extends beyond the mechanical gap end, causing undesired recording effects.
In addition, as shown FIG. 6, the region 7 that determines recording magnetization in the vicinity of the magnetic gap becomes curved, causing curved recording of a signal in that region 7 with respect to the magnetic head gap, as illustrated in FIG. 7. FIG. 8 shows the track pattern produced by recording with this magnetic head. Reference symbol .delta. in FIG. 8 indicates information which is recorded in the region 7. Since the azimuthal angle at which information is recorded in the region 7 differs from the mechanical azimuthal angle of the magnetic head, the portion indicated by .delta. is null in that it will not contribute to the reproduced output for that track. In video and digital audio applications where the track width is extremely small, the portions that do not contribute to the reproduced output are unduly increased, resulting in an overall smaller output level.